1. Field of the Invention
The present invention relates to a light-emitting and light-receiving assembly such as a semiconductor laser module or the like for use as a light source in optical communication and a method of manufacture thereof.
2. Description of the Related Prior Art
In the past years, semiconductor lasers for use as a light source in optical communication systems were designed into modules and optical fibers called "pigtail" used to convey laser light.
A conventional light-emitting assembly using a semiconductor laser as a light-emitting element is discussed with reference to FIG. 2 showing a cross sectional side view thereof. The light-emitting assembly in FIG. 2 includes a semiconductor laser 201, a lens 202, a light isolator 203, a package window 204, a core section 205 of an end surface of an optical fiber 212 for guiding light, a ferrule 206, a ferrule holder 207 and a package 208.
To construct the light-emitting assembly, a support 209 for the semiconductor laser 201 or "chip carrier" is mounted on a support 210 for the lens 202 or "lens mount." Then center-to-center adjustment is made in such a manner as to efficiently concentrate output light from the semiconductor laser 201 on the core section 205 of the end surface of the optical fiber. By the YAG welding technique, the ferrule 206 is fixed on the ferrule holder 207, which in turn is finally secured in the package window 204.
Within the above described optical system consisting of the single lens 202, the number of apertures or "NA" of the output light from the semiconductor laser 201 is usually within a range from 0.4 to 0.6; the NA of the output light from the optical fiber 212 is 0.1. It is therefore necessary to convert different NAs, because the single and common lens 202 must collect the output light from the semiconductor laser 201; it must also focus the laser light onto the end surface of the optical fiber 212 within the optical system consisting of only the single lens 202.
The optical system assumes a magnifying factor m as defined by equation (1) where L.sub.1 is the distance between the lens 202 and the semiconductor laser 201 and L.sub.2 is the distance between the lens 202 and the end surface of the optical fiber 212. EQU m=L.sub.1 /L.sub.2 ( 1)
As a result, the distance between the lens 202 and the end surface of the optical fiber 212 becomes longer than that of an optical system using a pair of lenses. Since the refractive index of optical components in the optical isolator 203 between the lens 202 and the optical fiber 212 is larger than that of air, the distance between the lens 202 and the end surface or optical fiber 212 is longer by a length of optical path corresponding to the difference in reflective index. Consequently, the semiconductor laser 201 and the end surface of the optical fiber 212 are usually secured and fixed on separate supports. Due to the difference in the thermal characteristics of support materials and thermal stress developed during the working and fixing of those supports, the optical axis of input laser light to the end surface of the optical fiber 212 may be shifted or dislocated under various environments during the actual use thereof.
As a solution to the above discussed problem, the improvement of a light-emitting assembly was proposed at the Third Optoelectronics Conference (OEC '90) Technical Digest, July 990, pp 216-217.
FIG. 3 is a cross sectional side view of the proposed light-emitting assembly. In FIG. 3, the light-emitting assembly includes a semiconductor laser 301, a lens 302, an optical isolator 303, a core section 305 of an optical fiber 312 for light transmission, a ferrule 306, a ferrule holder 307, a Peltier effect element 311 and a rod lens 313.
To complete the light-emitting assembly, a support for a spherical lens 302 or "can" is fixed on a support for the semiconductor laser 301 or "stem" and then the optical isolator 303 is fixed in such a manner as to minimize the insertion loss of the optical isolator 303 when the semiconductor laser 301 is driven. The rod lens 313 is attached to maximize the coupling efficiency and center-to-center adjustment is made to efficiently concentrate laser light on the core section 305 of the end surface of the optical fiber. The ferrule 306 is fixed in the ferrule holder 307 by the YAG welding technique and then the ferrule holder 307 is mounted on the optical isolator 303. Finally, the resulting unit is solder fixed onto a temperature-controlling Peltier effect element.
With the above described light-emitting assembly, center-to-center adjustment is made out to maximize output light from the spherical lens 302 with the semiconductor laser 301 in driven state, prior to the fixing of the can for the spherical lens 302 to the stem for the semiconductor laser 301. Such center adjustment may be accomplished only in a direction normal to the optical axis of the assembly and not in the direction of the optical axis, with difficulty in maximizing the coupling efficiency.
Therefore, the conventional light-emitting assemblies experience problems as follows. The semiconductor laser 201 and the end surface of the optical fiber 212 are generally secured on separate supports so that the optical axis of laser light incident on the end surface of the optical fiber 212 may be shifted or dislocated under various environments during the actual use due to the difference in thermal characteristics of the support materials and thermal stress internally developed in the support materials during the machining or fixing procedure thereof. The chip carrier for the semiconductor laser 201 is placed on the lens mount 210 for the lens 202 and thereafter the optical fiber 212 is adjusted into exact alignment with the focal point of focusing light from the lens 202. The positioning accuracy requirement when the chip carrier 209 is placed on the lens mount 210 is, however, increased m times in the direction of the optical axis and m.sup.2 times in a direction normal to the optical axis. Therefore, deviations of the center position adjustment are substantial and adjustment of this sort is time consuming. The inner diameter of the package window 204 should be large enough to accommodate such substantial deviations, impeding downsizing of the package.